Astigmatism is a refractive error along a specific meridian of the cornea; the image of an object falling in front or behind the retina in that meridian. Astigmatism has a power in diopters, and an axis denoting a direction on a 360-degree scale. The axis reflects the direction of the steepest or flattest meridian of the cornea.
Astigmatic surgery comprises flattening the steepest meridian or steepening the flattest meridian of the cornea. Using the excimer laser, the ablation profile is elliptical or butterfly-like according to the combination of the spherical and cylindrical refractive error. Several factors affect the optimal refractive outcome of astigmatic laser treatment. One of which is the exact alignment of the short axis of the elliptical ablation or of the butterfly-like ablation with the steepest meridian of the cornea in order to have the maximal corrective effect.
Astigmatism measurement is done while the patient is in a sitting position. However, astigmatism treatment is done while the patient is in a supine position. Several studies have detected ocular cyclotorsion, the eye turning around an axis passing through the center of the pupil, while the patient moves from the sitting to the supine position. If this eye movement is not taken into consideration when the patient lies under the laser, the direction of the short axis of the ellipse or of the butterfly-like ablation will not coincide with the steepest corneal meridian. Mathematically, missing the axis of astigmatism to be treated by 10 to 15 degrees leads to a treatment under-correction of 50%.
Currently, to detect any positional cyclotorsion of the eye, the refractive surgeons place a point mark at the 6 o'clock or the 3 and 9 o'clock axes of the pupil. This mark is put at the limbus of the eye, while the patient is sitting at the slit lamp. Under the laser, the alignment of the mark with either the 6 or 3 and 9 o'clock arms of a reticle incorporated in the visualization system of the laser is checked, while the reticle is kept centered on the pupil. If cyclotorsion occurs, the head of the patient is slightly rotated to align the laser reticle with the point mark in order to compensate for the eye movement.
The marking is done by free hand marking with a surgical ink pen after visually identifying the approximate limbal 6 o'clock position at the slit lamp. The prior art of free hand marking is unreliable, non-precise, and non-reproducible, with an error margin of 5 to 10 degrees leading to an estimated treatment under-correction between 18% and 35%.
Photorefractive keratectomy (PRK) and Laser in situ keratomileusis (LASIK) are the current laser refractive procedures. In PRK, the laser beam is applied to the corneal surface. While in LASIK a superficial round flap of the cornea is fashioned and lifted; then the laser beam is applied to the corneal bed beneath it. Subsequently the flap is flipped back to its original position, covering the treatment area. Using a marking dye, several radial marks are applied on the corneal surface before the flap is fashioned and lifted. These marks intersect the flap edges and ensure that the flap is repositioned to its original position by aligning them when the flap is flipped back at the end of the treatment. Traditionally, the marking of the corneal surface is done while the patient is lying under the laser. Furthermore, the marking of the corneal surface and the marking of the 6 o'clock position of the pupil are traditionally done in two separate procedures.
It is therefore an object of the present invention to provide improved cornea marker apparatus having marker blades that are conspicuous for purposes of radial placement to accurately determine and mark, on the cornea, the cardinal axes of the pupil (3, 6, and 9 o'clock axes) using a level.
It is another object of the invention to provide improved cornea marker apparatus having marker blades for radial placement to mark the surface of the corneal flap while the patient is sitting at the slit lamp.
It is still another object of the invention to provide an apparatus of the kind described having point markers to quantify the positional eye cyclotorsion.
It is yet another object to provide methods for marking the corneal surface while the patient is sitting at the slit lamp for purposes of corrective surgery.